Module 2 : Electrostatics
Lecture 11 : Conductors and Dielectric
  When a dielectric is placed in an electric field the dipoles get partially aligned in the direction of the field. The charge separation is opposed by a restoring force due to attaraction between the charges until the forces are balanced. Since the dipoles are partially aligned, there is a net dipole moment of the dielectric which opposes the electric field. However, unlike in the case of the conductors, the net field is not zero. The opposing dipolar field reduces the electric field inside the dielectric.
    Dielectric Polarization
 

Electric polarization is defined as the dipole moment per unit volume in a dielectric medium. Since the distribution of dipole moment in the medium is not uniform, the polarization $\vec P$is a function of position. If $\vec p(\vec r)$is the sum of the dipole moment vectors in a volume element $d\tau$located at the position $\vec r$,

It can be checked that the dimension of $\vec P$ is same as that of electric field divided by permittivity $\epsilon_0$. Thus the source of polarization field is also electric charge, except that the charges involved in producing polarization are bound charges .

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